1. Field of the Invention
The present invention relates to an image processing device, an image processing method and an imaging device, and particularly to an art for reducing false color (color moire) generated by synchronization processing on a mosaic image corresponding to a color filter arrangement arrayed on a single-plate imaging element.
2. Description of the Related Art
FIG. 8A illustrates an example of a color filter arrangement provided for a conventional imaging element, illustrating a primary color Bayer array which is most widely used.
The Bayer array is composed of two patterns of alternating lines, in which one pattern line consists of alternating G and R pixels out of three primary colors R (red), B (blue) and G (green) for pixels, and the other pattern line consists of alternating G and B pixels. The G pixels are arranged in a checkered pattern.
FIG. 8B is a diagram illustrating frequencies which allow reproduction in a mosaic image of a Bayer array.
When the pitch of a single pixel in the horizontal and vertical directions is defined as p, as illustrated in FIG. 8A, the Nyquist frequency of G in the horizontal direction H and in the vertical direction V is 1/(2p), and the Nyquist frequency of each of R and B in the horizontal direction H and in the vertical direction V is 1/(4p). Meanwhile, the Nyquist frequency of each of R, G and B in the diagonally upper right and diagonally lower right directions is 1/(2√2p).
When a subject image having a spatial frequency of 1/(4p) or more and less than 1/(2p) is inputted in a horizontal direction H or vertical direction V where G color images and R, B color images have different Nyquist frequencies each other, as described above, the phases of RGB color images become out of phase due to the chromatic aberration of the lens, leading to the generation of false color (color moire).
For example, an input image with a pattern of black and white stripes gets colored at its edge when out-of-phase RGB color images are produced by the effect of chromatic aberration. The colored edge appears to be color moire at high frequencies corresponding to spatial frequencies described above. Especially, such a problem, the appearance of false color is noticeable in an imaging device that does not have an optical low-pass filter (optical LPF) for reducing the generation of false color on the front of the light receiving surface of the imaging element.
A conventional image processing device corrects false color and color shifts by detecting a color shift generated by the chromatic aberration of a lens based on the correlation of two color components (R and G) and by scaling RGB color images according to the detected color shift, as is proposed in Japanese Patent Application Laid-Open No. 2010-045588 (PTL 1).
An image processing device described in PTL 1 shows the effect of reducing false color in input images which frequency is converted to a frequency lower than the Nyquist threshold frequencies of RB color images by its optical LPF. Unfortunately, it has been found out that the phases of RB color images become out of phase due to the chromatic aberration of the lens, leading to the generation of false color (color moire), when an input image having a spatial frequency of 1/(4p) or more and less than 1/(2p) is taken by an imaging device without an optical LPF.
In other words, correction cannot be made for chromatic aberrations generated in the neighborhood of a resolution with a Nyquist threshold frequency of 1/(4p) or more for RB color images and a Nyquist threshold frequency of less than 1/(2p) for G color images, among lens aberrations generated in a full frequency range.
As a conventional art for solving problems of this kind, an image processing method is proposed, as is described in Japanese Patent Application Laid-Open No. 2003-102025 (PTL 2). The image processing method described in PTL 2 allows you to extract a high-frequency component of a luminance component from a color signal of interest and detect the hue of the color signal of interest and the hues of peripheral pixels using pixels except for the pixels located in horizontal and vertical directions which present high possibility of false color generation. When the hue of the color signal of interest is within a predetermined hue region which is one of the characteristics of false color, and the extracted high-frequency component is in a high-frequency region, false color is reduced by making correction so as to bring the hue of the color signal of interest close to the hues of the peripheral signals.